Heterosexual transmission is the major mode of spread of human immunodeficiency virus (HIV) in Africa. Prevention of HIV transmission is an urgent public health challenge. The proposed studies will be conducted in Nairobi, Kenya, and are designed to enhance our understanding of the epidemiology, mechanisms, and prevention of HIV transmission. The first study will determine whether sexually transmitted diseases (STDs) which cause cervicitis or genital ulcers are associated with increased rates of isolation of HIV from the genital tract. HIV seropositive prostitutes attending an established STD research clinic will be enrolled in this prospective study. Secretions from genital ulcers and from the cervix during and after resolution of cervicitis will be obtained from HIV culture. If STDs are associated with enhanced HIV shedding from genital sites, then STD control programs will be an important component of AIDS control efforts. The second study is a prospective placebo-controlled trial to determine the efficacy of nonoxynol-9 impregnated contraceptive sponges during vaginal intercourse in reducing the risk of HIV infection. HIV seronegative prostitutes will be enrolled in the study, randomly assigned to the use of either nonoxynol-9 containing sponges or placebo vaginal suppositories, and followed for a one year period for HIV seroconversion. if nonoxynol-9 is found to significantly reduce the risk of HIV transmission, it is hoped that its use, in conjunction with condoms, will limit the spread of AIDS. The third study is a case-control study of risk factors for HIV infection in a low risk population of pregnant women. Seropositive and seronegative mothers enrolled in a separately funded study of congenital and perinatal transmission will be compared with respect to such potential risk factors as history of oral contraceptive use, prior genital ulcer disease, place of origin, number of sex partners, blood transfusions, and exposure to injections. If a correlation between oral contraceptive use and HIV infection is found, studies to explore the mechanisms of oral contraceptives serving as a cofactor for HIV acquisition will be planned.